Loading...
HomeMy WebLinkAboutFile Documents.13 E Water Pl.0008-2023-BREP (10) I--I P Hepworth-Pawlak Geotechnical,Inc. 5020 County Road 154 Ge .)tecl-"i Glenwood Springs,Colorado 81601 Phone:970-945-7988 HEPWORTH-PAWLAK GEOTECHNICAL Fax:970-945-8454 email:hpgeo@hpgeotech.com Revised March 23, 2010 March 10, 2010 City of Aspen Attn: Larry Doble <Larry.Doble@ci.aspen.co.us Job No. 110 030A Subject: Slope Stability Analysis, Steep Slope Between Castle Creek Road and Water Place Housing, Aspen, Colorado Dear Larry: As requested, we observed the subject slope and performed stability analysis at the subject site to evaluate the existing slope conditions and possible mitigation methods. The findings of our analysis and preliminary recommendations for the mitigation design are presented in this report. The services were performed in accordance with our agreement for professional engineering services to the City of Aspen, dated February 4, 2010. We previously conducted a subsoil study for design of foundations at Water Place Housing and presented our findings in a report dated May 8, 1995, Job No. 195 192. Slope Conditions: We observed the slope conditions on February 23, 2010. Overall, the slope faces east and is up to 90 feet high above Castle Creek Road. The slope has been over steepened by Castle Creek Road construction. No attempt to mitigate the steep slope has been performed resulting in a large escarpment which appears to be actively raveling down into Castle Creek Road. Jersey barriers have been placed on the uphill side of the road to catch rocks falling from the slope. The snow directly behind the Jersey barriers is about 3 feet deep. The lower two thirds of the escarpment slope is snow covered and very steep, estimated at about 45 to 50 degrees from horizontal. The upper third is clear of snow and near vertical, estimated at an average of 70 degrees with some areas that are steeper. The soils exposed in the snow clear, near vertical slope appeared to be glacial moraine deposits consisting of relatively dense, silty sandy gravel with cobbles and boulders. The top of the slope,just below Water Place Housing, is snow covered, vegetated with oak brush and slopes at 30 to 45 degrees. No water seepage from the slope was observed. Slope Stability Analysis: We analyzed the highest part of the slope which is directly below the portion of Water Place Housing closest to Castle Creek Road. The slope profile was taken from the aerial photograph with topographic lines that was provided to us. The stability of the existing natural slope was evaluated by a two-dimensional limiting equilibrium method computer program(SLOPE/W—Version 5) which calculates the factor of safety against instability by the Janbu simplified method of slices. We assumed that the soils in the slope consist of relatively dense sand and gravel with cobbles and Parker 303-841-7119 • Colorado Springs 719-633-5562 • Silverthome 970-468-1989 City of Aspen March 10, 2010 Page 2 boulders. Based on the slope of the lower escarpment and our experience with similar soils, we estimated that the angle of repose is similar to the slope indicating a strength angle of 50 degrees with an assumed moist unit weight of 135 pcf. The groundwater level was assumed to be about 7 feet below the ground surface in the middle of the Water Place Housing (based on our previous study) and linearly down to the ground surface at the toe of the slope next to Castle Creek Road. Analyzing the near vertical portion of the slope and assuming a factor of safety of 1.0 for that part of the slope results in an "apparent cohesion"value of 175 psf. The"apparent cohesion" may be caused by temporary, light cementing of the soil. Based on that cohesion value and a 50 degree strength angle, we analyzed the overall slope, which results in a calculated factor of safety of 1.0 for a slip surface that extends back to the eastern foundation walls of the nearest buildings. Subsequently, we analyzed two potential mitigation techniques which consisted of two rows of tiebacks placed just above Castle Creek Road or a 17 foot high mechanically stabilized earth (MSE) retaining wall • placed on the uphill side of Castle Creek Road. Discussion: Typically, factors of safety greater than 1.5 are considered adequate for slope stability. Factors of safety between 1.5 and 1.0 are considered temporarily stable but could become unstable due to erosion, rising groundwater table, additional loading or vibration from seismic events. Factors of safety below 1.0 indicate a high risk of potential failure. The actual values calculated in our analysis are based on assumptions including soil strength and location of water table averaged over the slope. We intended that these values be on the conservative side (resulting in lower factors of safety) while remaining reasonable for the observed conditions. The purpose of our mitigation analysis was to evaluate the potential increase in stability for the trial mitigation method. While the slope has probably been eroding since Castle Creek Road was built and probably widened, we expect it has been more or less stable with regard to massive slope failure for many years including some very wet periods and many freeze/thaw cycles. Eventually, the slope will continue to erode back to somewhere between 45 and 50 degrees, similar to natural slopes in the area. That slope will extend back into the eastern foundation elements of the easternmost Water Place Housing buildings. The most likely part of the slope for massive failure is the near vertical portion near the middle of the overall slope which is probably the most difficult part of the slope to access. In order to mitigate long-term progressive erosion of the slope which could potentially impact the nearest Water Place housing units, we analyzed two mitigation methods (retaining wall and tiebacks) Iocated at the bottom of the slope adjacent to Castle Creek Road which we considered relatively accessible. These two methods provide increased factor of safety for the overall slope (1.1 for the retaining wall and 1.3 for the tiebacks) but do nothing for the stability of the higher, steeper portions which are the most likely to fail. The third option is a micropile wall placed adjacent to Water Place Housing and constructed on that property. This option does nothing to improve the slope condition but does protect Water Job No. 110 030A Ptech City of Aspen March 10, 2010 Page 3 Place Housing in the event of massive slope failure or the progressive eroding of the slope. All three options would require a design/build specialty contractor. Retaining Wall: Placing a 17 foot tall MSE wall along Castle Creek Road would buttress the lower part of the slope and would provide a"landing area" for rockfall and a possible working platform off Castle Creek Road for continuing mitigation of the upper slopes. The increase in overall slope stability would only be about 10%. Placing a 1:1 reinforced fill slope above the retaining wall would increase the factor of safety against overall slope failure by about 20%but also greatly reduce the erosion potential. Construction would involve the County and would be disruptive to traffic on Castle Creek Road. Tiebacks: Placing two rows of 100 kip capacity tiebacks on 5 foot spacing at 5 and 10 feet above Castle Creek Road would result in an increase in stability of about 30% for overall slope failure but would do nothing for slope failures in the upper, near vertical portion of the slope. Placing 2 or 3 rows of tiebacks in the near vertical portion of the slope would greatly increase the stability of that area but would be difficult to construct if not impractical. Construction of some form of tiebacks would also involve the County and be disruptive to traffic on Castle Creek Road. Micropile Retaining Wall: It may be possible to construct a micropile retaining wall on Water Place Housing property just downslope of the two buildings closest to the slope which are about 20 feet off the property line. The wall would need to be about 80 feet long and would consist of a series of vertical and battered micropiles connected at the top with a concrete cap beam. We expect that the micropiles would be at least 40 to 50 feet deep. The wall would be buried completely and designed to be self supported in the event of progressive slope movement towards the buildings. This option would not involve the County or disrupt traffic on Castle Creek Road. There would likely be some inconvenience during construction to the residents of Water Place Housing. Additional subsurface information and further evaluation of a selected option or options should be obtained for design purposes. The recommendations submitted in this letter are based on our observation of the conditions exposed at the time of our site visit and the previous limited subsurface exploration at the Water Place Housing site. Variations in the subsurface conditions could increase the risk of slope movement. We should be advised of any variations encountered in the slope conditions for possible changes to recommendations contained in this letter. Job No. 110 030A COtech City of Aspen March 10, 2010 Page 4 If you have any questions or need further assistance, please call our office. Sincerely, HEPWORTH - PAWLI .PTCHNICAL, INC. get E Daniel E. Hardit P E 24443 s t ter. Reviewed by: SL o,� •, °•'�'�ti DEH/kac Job No. 110 030A G Etech